What Order units can give a Serahon Wizard a buff to its cast rolls? I know of the Slann, Astrolith brearer and the stormcast stardrake, but is there any more?
Those are the only ones. SCE are our sole allies and the only tool they have that isn't Stromhost or battalion specific is the Stardrake's Arcane Lineage. Sadly the Stardrake cost too many Ally Points to include unless you play +2500 MPP.
No units I know of. But arcane terrain would help. And there are one or two specific terrain pieces (Skullvane Manse and Arcane Ruins IIRC) that grant such a bonus. Edit: Witchfate Tor and Dreadstone Blight as well.
I forgot about special terrain. Normally at the events I attend the tables are all pre-built and you just roll for terrain affects at the start of the setup phase.
The thing is: special terrains are handy, but you cannot prepare a tactic upon them. You just use them if they are present. However, fielding a Slann, you can plan to have the constellation that gives another +1.